Package "debhelper" is required to run command "debuild" in 20.04

[John Liu]

I'm following the Packaging Guide and when I first try to run command `debuild -S -d -us -uc`, some error occurred:

```
$ debuild -S -d -us -uc
 dpkg-buildpackage -us -uc -ui -S -d
dpkg-buildpackage: info: source package hello
dpkg-buildpackage: info: source version 2.10-2johnliu1
dpkg-buildpackage: info: source distribution focal
dpkg-buildpackage: info: source changed by John Liu <johnliu@johnliu-ubuntu-2004-vm>
 dpkg-source --before-build .
 debian/rules clean
dh clean
make: dh: Command not found
make: *** [debian/rules:3: clean] Error 127
dpkg-buildpackage: error: debian/rules clean subprocess returned exit status 2
debuild: fatal error at line 1182:
dpkg-buildpackage -us -uc -ui -S -d failed
```

It turns out I need to install package "debhelper", but it's not listed in section "2.1. Install basic packaging software". I'm running minimal installed 20.04, not quite sure if it's related.